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Swan Falls, Part 2

The Snake River Canyon walls at Swan Falls are about 700 feet high. The river, greatly enhanced by the catastrophic Bonneville Flood, has eroded down through layer after layer of basalt, rhyolite and concreted ash. The 700 vertical feet of erosion hasn’t even made it a third of the way through the layers of volcanic rock. Swan Falls was a basalt shelf; Swan Falls Dam sits on uneroded basalt.

Snake River Canyon, Upstream at Swan Falls Reservoir

This is the view upstream, from near the canyon rim. The mountains in the distance are the Owyhees, a mix of volcanoes and basin and range. The scree slopes belong the cliffs are a mix of the usual scree and deposits from the Bonneville Flood.

The road down to the dam, descending into the canyon, is a trip back in time, down through the river cut into older and older rock.

Uppermost 150 Feet of North Side of Snake Canyon

There’s a lot going on there; let’s work our way through it. The top stuff is pillow basalt. It forms when molten rock meets water. Lava, flowing across the Snake River Plain, encountered water, a lof of water, enough to create 100 feet or so of pillow basalt. That would almost certainly be a lake created by a lava flow across the early Snake River, impounding water.

The layered, banded stuff at the bottom of the photo – still 600 feet from the river – is a variety of ash and cinders. Let’s take a closer look.

Detail of Ash and Cinder Layers

This is six feet of that ash layer. What you see is layer after layer of ash and cinders. The cinders imply the volcanic vent was fairly close; wind doesn’t blow cinders as far as ash. Reddish layers imply a time gap between eruptions, long enough for rain and air to leach some of the iron out of the ash. The bedding is pretty uniform, pretty remarkable with 100 feet or more of basalt is piled on top of it. What’s missing also tells you something: there are no black layers. That implies the eruptions occurred often enough that there was no opportunity for anything to grow on the ash; if there had been we’d see a thin layer of black carbon, burnt vegetation.

Basalt-Rhyolite Contact Zone

Where the basalt flows meet the ashy, rhyolite layers there are some interesting effects. The basalt is fractured and more permeable to the rain and melted snow; the rhyolite less so. So the water percolates down to the rhyolite and stops there. It oxidizes the iron in the basalt and ash, making them reddish. The bottom of the basalt is chemically altered as well, as the photo shows.